1. Field of the Invention
The present invention relates to a mobile communication system supporting an uplink packet data service. More particularly, the present invention relates to a method and apparatus for scheduling uplink data transmission for a User Equipment (UE) that uses an enhanced uplink dedicated transport channel.
2. Description of the Related Art
Universal Mobile Telecommunication Service (UMTS) is a 3rd generation mobile communication system that uses WCDMA and is based on the European Global System for Mobile communications (GSM) system. UMTS provides mobile subscribers a uniform service for the transmission of packet-based text, digitized voice, video and multimedia data at or above 2 Mbps irrespective of their geographic location. With the introduction of the virtual access concept, UMTS allows access to any end point within a network at any time. Virtual access refers to packet-switched access using a packet protocol like Internet Protocol (IP).
The UMTS system uses a transport channel called Enhanced Uplink Dedicated CHannel (EUDCH or E-DCH) in order to provide improved packet transmission performance for uplink communications from a UE to a Node B (or base station). To increase high-speed data transmission stability, Adaptive Modulation and Coding (AMC), Hybrid Automatic Repeat reQuest (HARQ), and Node B-controlled scheduling have been added to E-DCH transmissions.
AMC is a technique for adaptively selecting a modulation and coding scheme (MCS) according to channel conditions between a Node B and a UE. A plurality of MCS configurations can be defined in accordance with the available modulation and coding schemes. The adaptive selection of an MCS configuration according to channel conditions increases resource use efficiency.
HARQ is a packet retransmission scheme for retransmitting a packet to correct errors in a previously transmitted packet. HARQ comprises Chase Combining (CC) and Incremental Redundancy (IR). In CC, the retransmitted packet is in the same format as the previously transmitted packet, whereas in IR, the previously transmitted packet and the retransmitted packet are formatted differently.
Node B-controlled scheduling is a scheme in which a Node B determines whether to permit E-DCH transmission for a UE. When IE-DCH transmission is permitted, an allowed maximum data rate is determined and data rate information is transmitted to the UE. Based on the data rate information, the UE determines an available E-DCH data rate.
FIG. 1 illustrates an uplink data transmission on the E-DCH in a typical mobile communication system. Reference numeral 110 denotes a Node B supporting E-DCH and reference numerals 101 to 104 denote UEs using E-DCH. As illustrated, UEs 101 to 104 transmit data to Node B 110 on E-DCHs 111 to 114.
Node B 110 individually notifies UEs of E-DCH transmission being allowed by transmitting to the UEs scheduling grants and E-DCH data rate information, based on buffer occupancy information, requested data rate and channel condition information received from the UEs. This operation is called scheduling of uplink data transmission. The scheduling is performed such that the measured increase in Node B's noise does not exceed a noise increase threshold, thus enhancing total system performance. For example, low data rates are allocated to remote UEs, such as UEs 103 and 104, whereas high data rates are allocated to nearby UEs, such as UEs 101 and 102. UEs 101 to 104 determine their allowed maximum data rates for E-DCH data based on the scheduling grants and transmit the E-DCH data at the determined data rates.
The uplink signals of the different UEs interfere with one another due to asynchronization of the signals. Reception performance of a Node B increasingly suffers as the numbers of uplink signals increases. The compromised reception performance occurs when the numbers of uplink signals increases because as the numbers of uplink signals increases so does the amount of interference on the uplink signal of any given UE. This problem can be overcome by increasing the uplink transmit power of the UE. However, in doing so, the increased transmit power in turn serves as interference to other uplink signals. Thus, the reception performance would still be compromised at the Node B. The total power of uplink signals received at the Node B needs to be limited in order to maintain acceptable reception performance. Rise Over Thermal (ROT) represents uplink radio resources used by the Node B is defined asROT=Io/No  (1)where Io denotes a power spectral density over a total reception band, that is, the total power of all uplink signals received at the Node B. No denotes the thermal noise power spectral density at Node B. Therefore, an allowed maximum ROT represents the total uplink radio resources available to Node B.
The total ROT is expressed as the sum of inter-cell interference, voice traffic and E-DCH traffic. With Node B-controlled scheduling, simultaneous transmission of packets at high data rates by a plurality of UEs is prevented, thus maintaining the total ROT at or below a target ROT so as to ensure acceptable reception performance at all times. When high data rates are allowed for particular UEs, they are not allowed for other UEs in the Node B-controlled scheduling. Consequently, the total ROT does not exceed the target ROT.
In the case where many UEs are using the E-DCH service in one cell, the overhead of downlink signaling for scheduling grants must be considered in Node-controlled scheduling. For a large number of UEs using the E-DCH, the downlink power consumption of the Node B increases when transmitting scheduling grants and the number of downlink channelization codes increases to receive the scheduling grants. As a result, the whole downlink capacity of the cell decreases.
Accordingly, there is a need for a technique that reduces downlink signaling overhead when transmitting scheduling grants involved in Node B-controlled scheduling so as to increase downlink capacity.